Submitted in partial fulfillment of the requirements for the degree of
Doctor of Philosophy
at the Massachusetts Institute of Technology
and the Woods Hole Oceanographic Institution
September 2004

en

dc.description.abstract

This thesis addressed questions about the Fe cycle by measuring detailed profiles
and transects of Fe species in the ocean and also by exploring the use of a new tracer of
Fe, Fe isotopic fractionation. In the subtropical and tropical Atlantic Ocean, transects and
profiles are presented for dissolved Fe (<0.4 μm), soluble Fe (<0.02 μm), and colloidal
Fe (0.02 to 0.4 μm). Surface dissolved Fe distributions reflect atmospheric deposition
trends with colloidal Fe following dust deposition more strongly than the soluble fraction
of Fe. Observed surface maxima and shallow minima in dissolved Fe were always due to
variations in the colloidal Fe fraction. Deep-water dissolved and colloidal Fe
concentrations vary with water mass source, age, and transport path. Elevated dissolved
Fe concentrations (>1 nmol/kg) were associated with an oxygen minimum zone in the
tropical Atlantic at 10°N, 45°W.
Fractionation of iron isotopes could be an effective tool to investigate the
geochemistr of iron. Trace metal clean plankton tows, river samples, aerosol leachates,
and porewater samples were measured for their iron isotopic composition using a GV
Instruments IsoProbe Multi-collector ICPMS. The Fe isotopic composition of plankton
tow samples vared by over 4% (in 56Fe/54Fe). North Pacific plankton tow samples had
isotopically lighter Fe isotopic compositions than samples from the Atlantic. The overall
isotopic range observed in the Amazon River system was 1.5%, with variability observed
for different types of tributaries. The main channel river dissolved Fe samples and
suspended loads were isotopically similar (≈ -0.2 to -0.45% relative to igneous rocks).
The isotopically heaviest sample collected was dissolved Fe from an organic rich
tributary, the Negro River (+0.16%). In contrast, the suspended load from the Negro
River was isotopically light (-1 %). The isotopically lightest sample from the Amazon
region was shelf porewater (-1.4%). In river water-seawater mixing experiments, the Fe
isotopic signal of dissolved Fe of river water was modified by flocculation of isotopically
heavy Fe. The observed range in the Fe isotopic composition of the natural samples
including biological and aqueous samples demonstrates that significant and useful
fractionation is associated with Fe biogeochemistry in the environment.

en

dc.description.sponsorship

This research was supported by NSF grants OCE-0002273 and OCE-99871442.
The Amazon field trip was partially funded by the Houghton Fund at MIT. I was funded
by the National Physical Science Foundation, Lawrence Livermore National Laboratory,
and the Education Office of Woods Hole Oceanographic Institution.

Pathways of exchange between the shelf and slope in the Mid-Atlantic Bight were investigated
using a combination of radiochemical tracer and hydrographic measurements.
The motivation was to provide evidence of transport ...

This report is a follow-up to Woods Hole Oceanographic Institution
Technical Report WHOI-84-40. It contains 137cs and 90sr data from
seawater samples collected on four cruises in the northwestern North
Atlantic, Arctic ...

The goals of this thesis were: (1) to establish methods for the determination of
nitrogen and carbon isotope ratios in marine particulate and sedimentary
chlorophyll derivatives; (2) to establish chlorophyll δ15N and ...

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